Data taken by the Republic of China satellite (ROCSAT-1) during moderate to high solar activity years from 1999 to 2002 have been studied for the statistical distribution of the dominant light ion species, either hydrogen or helium ions, at 600 km topside ionosphere. The results indicate some interesting seasonal and longitudinal/latitudinal distributions of the dominant light ions in the topside ionosphere during the magnetic quiet periods. Each light ion species can become the dominant ion species at 600 km topside ionosphere but only at night when the ion temperature is cooler than during the day. More cases of H+ dominance have been observed than those of He+ dominance. Except for the March equinox the distribution of dominant H+ shows a strong hemispheric asymmetry for the other three seasons. When H+ dominance is observed in one hemisphere during the solstice season, the low latitude limit of this transition region is a constant dip latitude in the winter hemisphere. This statistical minimum of the transition latitude shows little dependence on the seasonal averaged solar flux intensity. Similar hemispherically asymmetric distribution for dominant He + in the winter hemisphere during the solstice season has also been noted except that the asymmetrical pattern is not as prominent as in the dominant H+ case because much fewer cases have been observed for dominant He+. The asymmetrical distribution of the dominant light ions seems to be related to the observed hemispheric field-aligned ion flow pattern. Thus it is concluded that the downward field-aligned ion flow together with the nighttime lower ion temperature in the winter hemisphere compose a possible cause for the occurrence distribution of the hemispheric asymmetry in the dominant light ion species. This can be understood from the fact that the field-aligned flow is related to the hemispheric asymmetry of the ionospheric F peaks and serves to enhance or retard the nocturnal redistribution of the light ions along the field line.